This invention relates to a process for producing a printed circuit board by an additive process.
Up to now, in the production of printed circuit boards by forming a circuit by electroless plating according to the additive process such as a so-called CC-4 process, an adhesive layer containing a catalyst for plating is previously formed on the surface of an insulating substrate. In this case, the insulating substrate may contain a catalyst for plating in some cases and may not contain in other cases. Further, in the case of a printed circuit board in which the insulating substrate is provided with holes for through holes, a treatment of adhering a catalyst for plating to the hole surfaces is carried out after forming the holes and before carrying out electroless plating treatment.
Usually, in order to improve the adhesion between the adhesive layer and the circuit of deposited metal formed by electroless plating treatment, the adhesive layer is roughened after adhering the catalyst for plating to the hole surfaces. The adhesive layer is roughened by immersing the insulating substrate in a roughening solution such as borofluoric acid solution or chromic anhydride-sulfuric acid type solution, but a seeder (catalyst) adhered to the hole surface is washed off in a large amount by this immersion treatment. Therefore, there is the following disadvantages: when the electroless plating treatment is subsequently carried out, it takes a long time to deposit a metal on the hole surface portions, and the plated film deposited adhere locally and thinly, so that plating voids tend to be formed. Accordingly, for example, when a land is subjected to solder plating with a solder dip or the like or when an electronic part is connected to the printed circuit board, the gas in the insulating substrate is blown off through fine voides of the plated film on the hole walls into the holes to bring about a state wherein vacancies are formed in the solder (hereinafter referred to as "blow-hole"). If such a state is brought about, there are the following disadvantages. Bad connection of the electronic part becomes liable to occur and the adhesion becomes liable to lower.
For example, U.S. Pat. No. 3,322,881 discloses a process which comprises first drilling or punching through holes in an insulating substrate, coating the whole surface including the lateral walls surrounding the through holes with resinous ink containing a catalyst for plating to form a first circuit pattern, masking the first circuit pattern leaving the lateral walls surrounding the through holes exposed, thereafter forming a second circuit pattern in the same manner as with the first circuit pattern, and then conducting electroless plating. In this process, as the resinous ink containing a catalyst for plating, there is used a resinous ink which has a high solid content in order to increase its adhesion to the substrate and the metal for plating, and has a viscosity as high as 5 to 100 poises (20.degree. C). Therefore, said process is disadvantageous in that the inner walls of the through holes cannot sufficiently be plated because the adhesion of the catalyst-containing resinous ink to the inner walls of the through holes becomes ununiform or in some cases, the holes are plugged. Further, the penetration of the ink into cracks formed on the through hole walls at the time of processing is insufficient, so that these defective portions cannot be repaired. Accordingly, said process is very insufficient in preventing the blow-holes. Further, said process also has the following disadvantage. When the catalyst-containing resinous ink is allowed to stand, the solvent such as xylene or diacetone alcohol volatizes, so that the ink increases in viscosity and lowers the workability, and the smaller the diameter of the through hole becomes, the more difficult becomes the ink to adhere to the through hole walls. Therefore, the process disclosed in this U.S. Patent is not suitable for the practical industrial production of printed circuit boards.